1. Field of the Invention
The present invention relates to power transmission and distribution systems, more particularly to devices for detecting and transmitting current and voltage fault information from power transmission and distribution lines to a control center, switching center or other designated ground station.
2. State of the Art
In the field of power transmission and distribution, generating systems produce electrical power which is transmitted through a grid of electrical high voltage alternating-current (AC), three-phase power lines. Occasionally, a transmission or distribution power line experiences a fault in which, for example, a short circuit or equipment failure on a power line causes a circuit breaker to trip open, causing a power interruption to the customer. Other faults, in particular voltage faults, can occur when a power line falls onto a high impedance surface, such as dry grass or an asphalt road, but the wire remains energized because the short circuit current is not high enough to trip the circuit breaker. Another type of voltage fault occurs when there is an open circuit on one of the power lines in a three-wire, three-phase system, causing extremely low voltage to the customers located beyond the fault. Another type of voltage fault is an open wire on one of the power lines in a four-wire, three-phase system, causing loss of voltage potential on the open phase beyond the open point.
Current faults can occur on the power line, such as inrush current caused by the starting of large motors. Deep voltage sags can occur when there is a remote line failure. Early detection and characterization of these faults in an electrical power transmission and distribution system are essential to a quick resolution of the problem and to future planning of upgrades to the power transmission and distribution system.
Some fault events can lead to low-or-no voltage brownouts or blackouts, while other faults create extremely hazardous conditions to the public. Other power line disturbances can cause problems to a customer who has very sensitive equipment that could be tripped off line by such disturbances as a voltage sag or a momentary interruption.
Prior art fault detectors have been developed in which various operating parameters of a power line are compared to present parameters to determine the character and amount of deviation. Some detectors have been used with event recorders in which analog representations of the parameters of interest are recorded and/or displayed.
Other prior art fault sensors or devices can only sense and record abnormal current that causes a sustained outage. Most often the device needs to be manually reset at the device's location. In addition, these prior art devices are not provided with data transmission capability. To identify the power line section with a fault, each prior art device needs to be visually inspected, which often takes hours to complete and prolongs outage time for the affected customers.
Retaining sensor integrity over the life of the sensor is key to minimizing operation and maintenance cost. In the prior art, in order to monitor line current and voltage and transmit information relating to the faults occurring on a transmission or distribution line, batteries were needed to power a microprocessor and transmitter. The problem arises with such prior art devices that the batteries became exhausted over a period of time and require replacement or recharging. To manage such battery maintenance and/or replacement at thousands of remote sensing locations has involved considerable expense and has often resulted in a failure to detect faults over significant periods of time.
Other prior art devices mounted on an overhead transmission or distribution line derive power by utilizing the energy stored in the magnetic field surrounding the operating conductor. At locations such as at the end of a distribution line, the load current may be at such a low level that there is not enough energy to power the fault sensor. Such prior art devices utilize a magnetic iron collar surrounding the transmission or distribution line for extracting the magnetic field energy as shown in U.S. Pat. Nos. 4,635,055, 4,728,887 and 4,808,917. These collar devices are relatively bulky, expensive, heavy and difficult to install. Moreover, for satisfactory operation, it is necessary for the iron magnetic collar to be completely closed around the conductor without any gap, to provide flux continuity around the transmission line. This closed collar arrangement is necessary both in order to derive adequate power and also to isolate the measurement of current from the effect of other nearby conductors. Such closed iron core clamp devices are heavy and difficult to install, requiring special tools and in some cases two workers.
Another problem with the prior art fault sensors is that they are not remotely programmable. Thus, programming instructions and/or calibrations made prior to installation cannot be changed without retrieving or removing the device from the power line.
It is therefore one object of the present invention to provide a sensing device that can be attached to a transmission or distribution power line and which will sense and transmit at least one alarm condition using circuitry which is operated by power not dependent on power flowing in the overhead transmission or distribution power line.
Another object of the invention is to provide an overhead sensing device for a power line that can be attached to an overhead power line by a simple clamp on a shotgun hot stick, measure at least two power line operating characteristics, and transmit any fault information and/or alarm condition to a designated ground station using a transceiver powered by a bank of double-layer capacitors.
Another object of the invention is to provide an underground sensing device that can be clamped on the power line and that can measure at least two operating characteristics and then transmit any fault information and/or alarm condition to a designated ground station using a transceiver and a fiber optic link.
Another object of the invention is to provide a sensing device for an overhead power line that can be completely controlled or programmed by communication signals from the ground using a transceiver provided within the device.
Another object of the invention is to provide a compact, lightweight sensing device for an overhead power line that can be easily attached and removed from a power line and measure at least two of its operating characteristics.
Still another object of the invention is to provide a sensing device for a power line that can be attached to a power line for measuring at least two of its operating characteristics and that is particularly well adapted for economy of manufacture.